1. Field of the Invention
This invention relates to rewind springs and particularly to rewind springs for seat belt retractors.
2. Description of the Prior Art
The characteristics of the rewind spring on seat belt retractors has a significant impact on the comfort and convenience of the system to the user. Conventional rewind springs produce a torque which increases as webbing is extracted. Since sufficient retraction force is necessary to completely stow the webbing when the belts are unbuckled, many users find the resulting belt pressures on the body uncomfortable. This is particularly true when the parking torque must be initially high to lift relatively heavy buckle tongues having an adjust feature. In addition, the act of extracting webbing for purposes of buckling up is annoying and difficult for some when these forces continue to increase.
To counteract some of these complaints, numerous tension eliminating and reducing devices have been suggested by the industry to provide comfort to the user when the belt is worn. However, such retractors are made more complicated, and in many of the designs the user must intentionally manipulate the webbing or actuate a push button to obtain the desired comfort. This detracts from an ideal situation in which the desired features are provided automatically.
A negator type of rewind spring is known to produce a substantially constant force of extraction/retraction characteristics, such as is described in U.S. Pat. No. 3,251,627 to Fisher. However, the cost benefit of such devices has not been sufficient as many users still consider the retracting forces in the wear zone to be uncomfortable. Likewise, it is known that by gradually varying the free coil diameters of the rewind spring, the extraction/retraction forces can be made to gradually decline with the amount of webbing extracted, as discussed in U.S. Pat. No. 4,253,620 to Takai, et al. Although this approach has some advantages, it will be recognized that during extraction the drop-off of force is so gradual that the wearer does not fully appreciate the added convenience. Likewise, in the wear zone, the retracting forces are not reduced to the maximum potential due to the gradual change in retracting force, i.e., a small person would feel greater forces than larger persons, as the retraction forces tend to be inversely proportional to the amount of webbing extracted.
Accordingly, it would be desirable to provide a rewind spring that produces the required parking force and yet provides significant convenience to the user when donning the belt. In addition, it would be desirable to provide a rewind spring that in the wearing zone, the retraction forces would keep the webbing against the body of the user with minimum force possible and still maintain that force as a constant regardless of occupant's size or seat position.